Wayne, Thank you for your very comprehensive mail, it certainly has put my mind at rest re potential issues involving separate receive antennas into the K3.
73 Stewart G3RXQ On Sun, 3 Feb 2008 14:42:19 -0800, wayne burdick wrote: > Hi all, > > I'll address both of these issues. > > 1. RX ANT input signal handling during TX > > The KXV3's ANT IN jack includes a carrier-operated relay circuit (COR). > If the COR kicks in too soon, they relay will switch off/on during > keying. So its threshold is set high. It will still serve its primary > function, which is to protect the transceiver when unsafe signal levels > are present, such as when the RX antenna is in the near field of a KW > transmit signal, or when a transmitter is accidentally connected to > this port. > > This leaves a gray zone where the COR doesn't kick in, but the transmit > signal may still be very high. High-power contest stations sometimes > use additional external T/R switching if they can't avoid the use of > closely-coupled receive and transmit antennas. > > We recently to a close look at this gray zone, and found that we could > make an "RX ANT jack on steroids" by adding two more diode isolation > sections. The resulting parts count is considerably higher, but we feel > that the added protection during transmit would be worthwhile. So this > change will be included in production units (once we use up our present > supply of assembled KXV3 modules). > > Meanwhile, if your antenna configuration and power level warrant this > higher degree of isolation, and you already have a KXV3, you can modify > it. We're designing a small add-on board to make this easier. It will > be supplied free of charge, on request. > > This unit has not yet been assigned an Elecraft part number, and isn't > available yet, so PLEASE don't request it until we announce the > details. > > 2. Buffered I.F. output spectra > > The pick-off point for the K3's buffered I.F. output -- right at the > output of the 1st mixer's post-amp stage -- was selected to give the > widest possible bandwidth. Thus there is no filtering of any kind from > here to the BNC jack on the KXV3. This creates the opportunity to do > *very* wide spectral sweeps, if required -- limited only by the > bandwidth of the K3's band-pass filters. > > As some (armed with spectrum analyzers) have noted, this output signal > includes the usual spectra one would expect from a high-level mixer. > (And it *is* a very high-level mixer.) > > A mixer's output includes a broad range of products relating to the sum > and difference of its inputs (RF and LO). It also includes products > derived from harmonics of the sum and difference, etc. Collectively, > these will appear as a "comb" pattern on an analyzer. This is why the > mixer must be followed by band-pass filtering. (In transmit mode this > function is performed by the K3's narrow ham-band band-pass filters; in > receive mode, by the I.F. crystal filter and DSP filtering.) > > But the buffered I.F. output is at a different point in the signal > chain. So any filtering needed for a panadapter must be included either > in the panadapter itself, or in series with it. Some panadapters will > require no filtering at all, while some will need various degress of > L-C filtering (the L-C filter used on our I.F. noise blanker is a good > starting point). > > If you were only interested in a narrow band sweep, you could use a > ceramic or crystal filter. Our FM crystal filter, at about 13 kHz wide, > would be a good choice here. > > Excellent shielding should be used, and possibly additional buffering, > to prevent re-radiation of 8.215 MHz back into the K3. > > A related topic is the exact frequency of the buffered I.F. output. It > is nominally 8.215 MHz with our 8-pole filters, and typically 0.8 to > 0.9 kHz lower with our 5-pole filters. In addition, the portion of this > passband actually present at the audio output is determined by the DSP > controls (SHIFT, WIDTH, etc.). We can work with panadapter suppliers to > provide this data via the RS232 port, and of course any future Elecraft > panadapter would also have this information available. > > I'll be adding this subject to the Owner's Manual. If you're > interfacing a panadapter to the K3 and have any further questions, feel > free to e-mail me directly. > > 73, > Wayne > N6KR > > > --- > > http://www.elecraft.com > > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
